Add protection from min/max macros

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@145407 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/valarray b/include/valarray
index 5d3154c..3ac9494 100644
--- a/include/valarray
+++ b/include/valarray
@@ -346,6 +346,8 @@
 #include <algorithm>
 #include <functional>
 
+#include <__undef_min_max>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif